add Rust By Example to the bookshelf

src/doc/index.md

@@ -28,6 +28,7 @@ Rust provides a number of book-length sets of documentation, collectively
 nicknamed 'The Rust Bookshelf.'
 
 * [The Rust Programming Language][book] teaches you how to program in Rust.
+* [Rust By Example][rbe] teaches you how to program in Rust using editable examples.
 * [The Cargo Book][cargo-book] is a guide to Cargo, Rust's build tool and dependency manager.
 * [The Unstable Book][unstable-book] has documentation for unstable features.
 * [The Rustonomicon][nomicon] is your guidebook to the dark arts of unsafe Rust.
@@ -51,6 +52,7 @@ before this policy was put into place. That work is being tracked
 [refchecklist]: https://github.com/rust-lang-nursery/reference/issues/9
 [err]: error-index.html
 [book]: book/index.html
+[rbe]: rust-by-example/index.html
 [nomicon]: nomicon/index.html
 [unstable-book]: unstable-book/index.html
 [rustdoc-book]: rustdoc/index.html